The primary object of this invention is to provide hydraulic cementitious compositions which have improved compressive and tensile strength, and improved bonding properties. This improvement is accomplished by the incorporation into the cementitious formulation the pozzolan silica fume. Silica fume is the by-product, now a waste product, obtained during the manufacture of metallic silicon and ferrosilicon alloys. A pozzolan is by definition a product which reacts with lime at room temperature in the presence of water. Lime is generated during curing of the cement. The use of an efficient pozzolan improves the strength, the density, resistance to water penetration in the cured compositions, and insolubilizes the water-soluble calcium hydroxide. Silica fume is an excellent pozzolan and bonding agent due to its physical and chemical properties as described below.
The process of producing metallic silicon consists of feeding a charge of quartz, coal and wood chips to the top of a tall furnace, provided with carbon electrodes extending to the bottom of the furnace. At the high temperature at the bottom, the silicon dioxide is reduced by the carbon to silicon, which melts and is tapped periodically. To produce the ferrosilicon alloys, scrap iron is also added to the charge.
The gaseous by-products are carbon monoxide, which oxidizes to carbon dioxide, and silicon suboxide which oxidizes to silicon dioxide, when the gases reach the top of the furnace. The particulates are the silica fume.
Typical analyses and properties of the submicron silica fume are:
______________________________________ SiO.sub.2 96.09% Fe.sub.2 O.sub.3 0.34% MnO 0.09% Al.sub.2 O.sub.3 0.21% CaO 0.35% MgO 0.23% K.sub.2 O 0.59% Na.sub.2 O 0.07% SO.sub.3 0.35% loss on ignition 1.68% Surface area 25.9 m.sup.2 /gram Particle size 0.25-0.02 microns Average particle size 0.12 microns Bulk density as generated 4-6 pounds pcf Bulk density (packed) 12-14 pounds pcf ______________________________________
The extremely fine particle size and high concentration of silicon dioxide in the fume give it the high reactivity towards lime and thus its effectiveness as a pozzolan. Its presence in the cementitious compositions also greatly improves its bonding properties to aggregates and substrates, thus further increasing the strength properties of the compositions as well as providing an efficient mortar.